"That which does not kill me will make me stronger" might well have been the motto that Horslips adopted after the release of The Unfortunate Cup of Tea, their 1975 release. That album, generally regarded as their weakest, somehow gave rise to this, perhaps their strongest. Returning to their original formula of rock & roll, folk, and prog rock, Book of Invasions rivals 1973's The Tain as their most consistent and creative, and established the band as that decade's preeminent purveyor of those three entwined genres. Neither Fairport Convention nor Steeleye Span rocked as convincingly or consistently as they did, and Jethro Tull's fleeting folk fancies didn't qualify them as a folk-rock outfit. Often compared to Ian Anderson's group, Horslips furthered that notion with the opening chord progression of "The Power and the Glory," which is quite reminiscent of Tull's "Locomotive Breath." John Fean's electric guitar playing (especially when in tandem with Barry Devlin's bass) frequently conjures up Martin Barre comparisons. But it would be unfair to infer that this album is anything less than an original. Along with the group's first two albums (Happy to Meet Sorry to Part and The Tain) Book of Invasions: A Celtic Symphony set the standard for how next-generation Celtic rock albums would be judged. The degree to which they incorporated fiddles, concertinas, accordions, mandolin, tin whistles, and uilleann pipes into a rock & roll band was unprecedented in the '70s. This record was and is their crowning achievement.
AllMusic Review by Dave Sleger